The present invention relates to a developer deterioration detecting device which detects the deterioration of developer held in a developing means installed in an image forming apparatus which obtains an image in such a manner that: a toner image is formed on a photordceptor by an electrophotographic system; and the obtained toner image is transferred onto a transfer sheet.
Image formation by the electrophotographic system is conducted in such a manner that: a latent image corresponding to a document image or image data is formed on a photoreceptor; the formed latent image is developed so that a toner image is formed on the photoreceptor; and then the toner image is transferred onto a transfer sheet.
In a developing means which visualizes the latent image formed on the photoreceptor, one-component developer including only toner or two-component developer including toner and carrier is provided, and when the formed latent image is visualized, only toner is moved from the developing means to the photoreceptor so that a toner image can be formed on the photoreceptor.
In general, in the case of either one-component developer or two-component developer, the developer is stirred in the developing means in order to give an electric charge to the developer by means of frictional charging. In the case of a one-component developer, the amount of toner, which functions as developer, is detected by a sensor and controlled so that a constant amount of toner can be held in the developing means. In the case of two-component developer, the toner concentration, which is the ratio of toner to carrier, is measured by a toner concentration sensor. When it has been detected by the toner concentration sensor that the toner concentration in the developing means is low, new toner is supplied to the developing means. In the manner described above, toner concentration in the developing means is controlled so that it can be maintained constant. As mentioned before, in the case of one-component developer, the amount of toner is controlled, and in the case of two-component developer, the concentration of toner is controlled. The reason why control is conducted in the manner described above, is to maintain the developing performance constant in order to form an image of high quality.
However, since the toner amount is maintained constant in the case of one-component developer and the toner concentration is maintained constant in the case of two-component developer, the deterioration of developer is caused in such a manner that: when toner consumption is small in a developing means, that is, when the visualization area is small in an image formation, a large amount of toner stays in the developing means; accordingly, the toner in the developing means is stirred over a long period of time; and as a result, deterioration of toner such as an increase in electric charge given to the toner and a decrease in fluidity, is caused.
In the case of a conventional image forming apparatus, consideration has not been given to the aforementioned deterioration in toner.
When toner is deteriorated, the fluidity is lowered, so that the toner can not be stably moved from the developing means to the photoreceptor. As a result, the following problems are caused: Toner image density is lowered. The density of the toner image becomes too high. Fogging occurs in the image. Character images become too bold. Especially, in the case of color image formation, reproducibility of color is extremely lowered, so that developing performance is lowered or becomes unstable and image quality is degraded.
In order to detect toner concentration, there is a toner amount detecting system in which inductance is utilized for detection. Toner concentration can be detected by the aforementioned system as follows. Since carrier contained in developer is magnetic, toner concentration can be detected with an inductance sensor having a coil installed in the developer. Specifically, toner concentration can be found as follows: consideration is given to the phenomenon in which permeability of developer is varied when the mixing ratio of toner to carrier varies according to the fluctuation in toner concentration; and toner concentration can be found y measuring the permeability of developer.
In the aforementioned method, output voltage of the aforementioned inductance sensor is compared with a reference voltage, and toner is supplied so that the output voltage of the inductance sensor can be the same as the reference voltage. As a result, toner concentration can be controlled to be constant. The aforementioned technique has been disclosed in the official gazettes of Japanese Patent Application Nos. 28305/1988 and 5299/1989.
FIG. 21 is a characteristic diagram showing an example of an inductance sensor. The sensor shown in the diagram is characterized in that: when toner concentration is lowered, output voltage of the sensor is increased. Output voltage can be adjusted when control voltage V.sub.c given to the sensor is changed. In the initial setting, control voltage V.sub.c is determined so that a predetermined voltage, for example 2 V, can be obtained when toner concentration is the same as the reference concentration, for example, 7%. In this diagram, control voltage is V.sub.c2, which is supplied continuously.
In a color copier, various colors are reproduced by superimposing 4 colors of yellow, cyan, magenta and black. Consequently, the toner concentration of each color must be accurately controlled in order to maintain the color balance.
While developer is being used, its apparent density (which is the weight of developer per unit volume) is varied. When stirring and circulation are repeated in a developing unit, the developer is compressed as compared to the initial state, although the reverse behavior is shown, depending on the kind of developer. In the aforementioned compressed state, the amount of toner per unit volume and that of carrier per unit volume are increased. However, toner concentration is detected only by the amount of carrier, so that the output of the sensor is increased and toner concentration is mistakenly judged to have been lowered.
Therefore, there is a possibility that toner is supplied in the developing unit and toner concentration becomes excessively high. Due to the foregoing, the density of the outputted image becomes too high. In this case, the developing condition of each color developer is different, so that the color balance is upset and color reproducibility is deteriorated.
An object of the present invention is to solve the aforementioned problems. It is a primary object of the present invention to provide a developer deterioration detecting method of an image forming apparatus in which the deterioration of developer can be simply and accurately detected in a manner in which the amount of toner supply is calculated as the toner consumption, utilizing a conventional toner supply means. Another object of the present invention is to provide an image forming apparatus in which the degradation of developing performance, image quality and color reproducibility can be forestalled.
A further object of the present invention is to realize an image forming apparatus in which toner concentration can be accurately detected and always maintained constant regardless of the developing condition of the image forming apparatus.